1. Technical Field
The present invention relates to a photoelectric switch capable of receiving a light emitted from a light projecting device via a light receiving device and executing a sensing operation by comparing information sensed from a quantity of received light with a threshold value.
2. Related Art
The photoelectric switch is a sensor that is able to sense a sensed target in a noncontact manner, and is utilized to sense whether or not the moving target is present on a production line in a factory, and the like. Normally the presence/absence of the target is decided by projecting a light onto a carrying route of the target and then sensing a reflected light or a transmitted light of the light. The transmission photoelectric switch utilizes that a quantity of received light of the light receiving portion is changed because a light projected from the light projecting portion is intercepted by the object that passes through an object sensing area, and can decide the presence of the target based on this change. The transmission photoelectric switch includes the projection head having the light projecting device such as LED (light emitting diode), or the like, and the reception head having the light receiving device such as PD (photodiode), or the like. The projection head and the reception head are arranged to oppose to each other, and a photoelectric sensor is formed between the projection head and the reception head.
As the type of the photoelectric switch, there are the transmission type, the diffuse reflection type, and the coaxial regression reflection type. In the transmission type out of them, the projection head and the reception head are constructed as a separate body. While, in the coaxial regression reflection type and the diffuse reflection type photoelectric switches, the projection head and the reception head are constructed integrally. The integral type possesses such an advantage that laying of the wiring is simplified rather than the separate type and the coaxial regression reflection type possesses such an advantage that a sensed distance becomes longer than the diffuse reflection type because the sensor and the reflector plate are arranged to oppose to each other and put the sensed object between them.
Also, as the light projecting device used in the projection head of the photoelectric switch, the devices employing LD (semiconductor laser) instead of LED have been developed. The LD is superior in optical characteristics to the LED, and thus a light source of the photoelectric sensor using the LD is smaller than the LED. Therefore, the spot can be narrowed, visibility of the spot can be improved, and workability in the fitting operation can be improved. Also, since a light quantity density is high, a large quantity of light can be obtained by the small spot. As a result, such advantages can be achieved that the sensing performance can be improved, and the like.
Meanwhile, the safety measures are required of the products handling the laser beam in response to the output beam according to various standards. For example, in “Radiation Safety Measure of the Laser Product” stipulated in JIS (Japan Industrial Standard), the safety measures are classified in response to the output of the laser beam of the laser product and the necessary safety measure is stipulated every class. In this manner, in the photoelectric sensor using the LD, the user must check according to standards of the safety whether or not a power of the laser beam projected as the sensing light is suppressed lower than a specified value in response to the country or the district, the service, and the class. Also, a quantity of emitted light of the laser must be suppressed lower than a predetermined level when a single failure occurs in the circuit. As the measure for this purpose, it may be considered that a coupling efficiency of the optical system should be decreased to in precaution against the case of failure such that, even when the laser output is increased to its maximum, a quantity of light emitted to the outside of the photoelectric sensor is lower than a specified value in Class 1. However, according to this method, few problems arise in the transmission photoelectric sensor when a quantity of emitted light is set in view of the lifetime of LD, nevertheless a quantity of projected light is small in the diffuse reflection type and the performance is extremely deteriorated.
Therefore, in the photoelectric sensor using the LD in the prior art, a protecting circuit for monitoring and limiting a quantity of light of the LD is provided on the projection head side. A configurative example of the photoelectric sensor having the protecting circuit is shown in a block diagram of FIG. 22. The photoelectric sensor shown in FIG. 22 is the separate type photoelectric sensor in which a controller 200D and a projection head 400D are constructed separately. The projection head 400D includes an LD as the light projecting device, an LD driving circuit 960, a power controlling circuit 956, and a monitor signal generating circuit 962. The LD driving circuit 960 drives the LD by connecting the LD and a monitor PD as the PD monitoring light receiving device. The power controlling circuit 956 supplies a power to the LD driving circuit 960. The monitor signal generating circuit 962 senses/amplifies a monitor current of the monitoring light receiving device. In the controller 200D, an LD timing controlling circuit 958 for sending out a lighting timing to pulse-drive the LD. Normally the LD has the monitoring light receiving device such as the monitor PD, or the like in the module, and thus a quantity of emitted light of the LD can be sensed by sensing a quantity of light of the monitoring light receiving device. Therefore, a feedback control is executed such that the LD can maintain a predetermined output by amplifying a quantity of emitted light and adjusting a gain by means of an APC circuit 955, and the like.
(Protecting Circuit)
In addition, the projection head 400D includes a monitor signal level deciding circuit 964, a monitor signal generating circuit failure deciding circuit 966, a DC lighting preventing circuit 970, and the like, as the protecting circuit. The monitor signal level deciding circuit 964 decides whether or not a monitor signal obtained by the monitor signal generating circuit 962 reaches a predetermined level (threshold value) and instructs the power controlling circuit 956 to control a power or cut off a power when the monitor signal reached the predetermined value. The monitor signal generating circuit failure deciding circuit 966 senses the failure of the monitor signal generating circuit 962. The DC lighting preventing circuit 970 monitors whether or not the pulse lighting of the light projecting device is switched into the DC lighting. An example of such protecting circuit is shown in a circuit diagram of FIG. 23. The projection head 400D shown in FIG. 23 includes an APC circuit block 955B, a monitor signal level deciding circuit block 964B, a monitor signal generating circuit failure deciding circuit block 966B, and a DC lighting preventing circuit block 970B.
(APC Circuit Block 955B)
The APC circuit block 955B turns ON/OFF an LD module 968, in which the LD as the light projecting device and the monitor PD as the LD monitoring light receiving device are built, the pulse signal. More concretely, an amplifier AMP1 turns ON/OFF a transistor Q2. A signal obtained by voltage-dividing a pulse signal fed from a pulse signal input terminal by a resistor R1 and a variable resistor (trimmer) R2 is input into the (+) side of the amplifier AMP1. Also, the (−) side of the amplifier AMP1 is connected to an output of the amplifier AMP1 via a capacitor C5 and connected to the output signal side of the monitor PD to feed back, and is grounded via a resistor R5. The output signal side of the amplifier AMP1 is connected to a base of the transistor Q2. Also, the emitter side of the transistor Q2 is connected to an anode of the LD via a protection resistor R4. Thus, when the pulse signal is input, the transistor Q2 is turned ON to drive the LD. A drive voltage terminal Vcc is connected to the collector side of the transistor Q2 via a transistor Q1. Transistors Q3, Q4, Q5 are connected to a base of the transistor Q1 via a resistor R3. All transistors Q1, Q3, Q4, Q5 are turned ON in the normal operation. In order to turn OFF the LD when the abnormality such as the failure occurs, a current supply to the LD is interrupted by turning OFF the transistor Q1. In order to turn OFF the transistor Q1, a base side voltage Vb of the PNP bipolar transistor Q1 is set higher than an emitter side voltage Ve by −0.6 V or more.
(DC Lighting Preventing Circuit Block 970b)
An emitter of the PNP transistor Q3 is connected to a base side of the transistor Q1 via the resistor R3 and a grounded capacitor C1 to constitute the DC lighting preventing circuit 970. The pulse signal input terminal is connected to a base side of the transistor Q3. When the pulse signal is kept at HIGH due to any abnormality, the LD is shifted from the pulse lighting to the DC lighting. In this case, the transistor Q3 is turned OFF, then the capacitor C1 is charged in the direction from the emitter of the transistor Q1 to the base via the resistor R3, and then the base side voltage Vb of the transistor Q1 becomes higher than the emitter side voltage Ve by −0.6 V or more. As a result, the transistor Q1 is turned OFF to interrupt a Vcc supply to the LD and thus the output of the LD is stopped.
(Monitor Signal Level Deciding Circuit Block 964b)
While, the monitor signal level deciding circuit block 964B is connected to the side on which the resistor R5 is connected to the anode of the monitor PD. In this monitor signal level deciding circuit block 964B, the resistor R5 is connected to the (−) side of a comparator CMP1. The (+) side of the comparator CMP1 is voltage-divided by a resistor R6 and a trimmer R7, and grounded via a capacitor C2. The output side of the comparator CMP1 is connected to a base of the transistor Q4. The trimmer R7 is adjusted such that the output of the comparator CMP1 is turned ON in the normal operation. This monitor signal level deciding circuit block 964B corresponds to the single failure of members except the resistor R5. The monitor PD of the LD generates a monitor current linearly in answer to the optical output of the LD. The monitor current generates a monitor voltage across the resistor R5 as a voltage drop. When this monitor voltage exceeds a predetermined value defined by a voltage division between resistors R6 and R7, the output of the comparator CMP1 is turned OFF to turn OFF the transistor Q4. The charge of the capacitor C1 is not pulled out as above when the transistor Q4 is turned OFF, so that the transistor Q1 is turned OFF to interrupt the supply of Vcc and thus the output of the LD is forcedly turned OFF.
(Monitor Signal Generating Circuit Failure Deciding Circuit Block 966B)
Further, the output side of the amplifier AMP1 in the APC circuit block 955B is connected to the (−) side of a comparator CMP2 constituting the monitor signal generating circuit failure deciding circuit block 966B to respond to the failure of the resistor R5. The (+) side of the comparator CMP2 is voltage-divided by resistors R8, R9 and is grounded via a capacitor C3. Also, an output of the comparator CMP2 is connected to a base of the transistor Q5. Accordingly, even when the monitor signal level deciding circuit 964 does not function because of the short-circuit failure of the resistor R5, the output of the amplifier AMP1 is monitored by the comparator CMP2, so that the transistor Q5 is turned OFF to interrupt a current supply to the transistor Q2 and thus the output of the LD can be stopped. That is, when the resistor R5 is broken down by the short-circuit, the monitor voltage of the APC circuit block 955B is still kept at 0 V and therefore the output of amplifier AMP1 generated based on the monitor voltage goes to the maximum voltage. The comparator CMP2 senses that the output of amplifier AMP1 had the maximum voltage, and turns OFF the transistor Q5. The charge of the capacitor C1 is not pulled out as above when the transistor Q5 is turned OFF, so that the transistor Q1 is turned OFF and thus the output of the LD is forcedly turned OFF.
In this case, the output sides of the comparators CMP1, CMP2 are connected to a resistor R11 and a capacitor C6 and a resistor R12 and a capacitor C7 respectively. Thus, the OFF state is maintained while the voltage value generated at a time of abnormality is held by the capacitors for a predetermined time.
As described above, in the photoelectric sensor using the LD in the prior art, since the control required to fulfill the safety standard is executed on the projection head side, various protecting circuits must be provided to the projection head. As a result, there were problems such that the number of articles is increased, a circuit configuration becomes complicated, and a size of the projection head is increased. Therefore, the photoelectric sensor in the prior art did not meet the demand for a further size reduction of the recent photoelectric sensor (JP-A-2003-086830, for example).